Cast iron has the characteristics of wear resistance and high strength, and is widely used in industries such as textiles, automobiles, electric power, home appliances, shipbuilding, and national defense. Its wide application in production practice directly affects the quality and performance of downstream products. Sulfur in cast iron can affect the quality of cast iron. How do we control the sulfur content in cast iron? This article can provide some inspiration.
Ferro Sulfur & Sulfur Cored Wire
The common sulfur-increasing agents are ferro sulfur, ferrous sulfide, ferro sulfur ball, and sulfur cored wire.
The Specification of Sulfur Cored Wire
S. – 98% MIN.
Size – 13MM Vertical Clipping.
Powder Weight – 200 (+/-)10gms/mtr.
Liquid iron is easy to increase sulfur, and the absorption rate is generally above 90%. Common desulfurizers include soda, lime, calcium carbide, magnesium, and calcium-based composite types. The porous plug pneumatic method combined with calcium-based desulfurizers to treat ductile iron molten iron has the characteristics of low energy consumption and high efficiency. It is very suitable for both cupola furnaces and induction furnaces. A foundry company with joint smelting.
The Function of Sulfur in Cast Iron
Surface active function
Normally, C, Si, Mn, and P in molten iron have little effect on surface tension. Free sulfur and free oxygen are surface active elements. With the increase of their content, the surface tension of molten iron decreases rapidly, and the wetting ability is greatly enhanced. The solubility of free sulfur is dozens of times that of oxygen, and it is very easy to react with Mn in molten iron, Mg, RE, Ca, Ba, etc. introduced by nodulizers and inoculants to form sulfides. most influential. Therefore, controlling the free sulfur content can indirectly control the graphite morphology.
It is believed that the desulfurization and deoxidation of spheroidizing elements reduce the free sulfur and oxygen content in molten iron, increase its surface tension, and thus affect the graphite morphology during eutectic transformation. When the carbon equivalent is constant, the increase of sulfur reduces the solubility of carbon in molten iron, which may be an element that promotes graphitization from a thermodynamic point of view, but it is an element that strongly hinders graphitization from a kinetic point of view. Due to the surface activity, free sulfur is enriched in the boundary of the eutectic group in the molten iron, blocking the dislocation steps, hindering the diffusion of carbon atoms and the growth of the graphite-austenite eutectic group, on the one hand, reducing the branching of precipitated graphite, to generate short and thick flake graphite, on the other hand, increase the supercooling degree of molten iron, so that the solidification process will change according to the metastable system.
Material chemical composition and mechanical properties
C and S are analyzed chemically, and the remaining elements are analyzed by spectrum. The test results of chemical composition and mechanical properties show that when the tensile strength and hardness meet the standard, the graphite form is very unstable. Generally speaking, the carbon content is above 3.4%. The growth trend of A-type graphite is more obvious. However, due to the high carbon equivalent, it can be clearly seen from schemes 1 to 8 that as the carbon equivalent increases, the tensile strength gradually decreases, hovering between 250 and 260 MPa.
Inhibition of nitrogen absorption function
Studies have found that sulfur can significantly reduce the rate of nitrogen uptake, and when the sulfur content is low, every 0.05% increase in sulfur will reduce the dissolution rate coefficient of nitrogen by about 50%, and sulfur has basically no effect on the solubility of nitrogen. Therefore, when using more scrap steel to produce gray cast iron, proper control of sulfur content can help inhibit nitrogen absorption.
Mechanism of sulfur increase
The microstructure of high carbon equivalent gray cast iron is characterized by coarse graphite, less austenite dendrites, and ferrite in the matrix structure. Generally, reducing the content of C and Si in cast iron and reducing the amount of graphite can improve the strength of cast iron. However, the reduction of C and Si content will induce the precipitation of D-type graphite, which is easy to produce white spots, and this is precisely to be avoided.
Similarly, castings need to be thinned. On the one hand, the strength of the material itself is required to be further improved, and the whitening tendency of the structure due to the increase in cooling rate must be prevented. The relationship between the structure parameters and properties of cast iron is an indeterminate correlation. , To solve the problem of high strength and thin wall of cast iron.
From the point of view of physical metallurgy, it is necessary to reasonably control the four relatively important organizational factors under the condition of higher carbon equivalent, namely, increasing the number of dendrites, refining eutectic clusters, refining graphite and strengthening the matrix.
It is generally believed that sulfur is an element that strongly stabilizes cementite and hinders graphitization in inoculated cast iron, but from the perspective of thermodynamics, sulfur can reduce the solubility of carbon in molten iron, enhance the activity of carbon, and form MnS with manganese and rare earths. And RES will become the core of non-spontaneous nucleation of graphite, so it can promote graphitization, so sulfur is an indispensable component to promote the smooth progress of inoculation reaction.
Therefore, sulfur has a dual role in cast iron: when the amount of sulfur is low, the effect of inoculating elements cannot be exerted, and the inoculation effect is not good; when the amount of sulfur is too high, due to the decrease in the atomic ratio of inoculating elements and sulfur, RE2S3 and RE3S4 will be formed Types of sulfides, they cannot be effective substrates for graphite nucleation, leading to deterioration of the inoculation effect.
Conclusion
Sulfur exists in the form of free sulfur, sulfide, and sulfur inclusions in molten iron. Free sulfur reduces the surface tension of molten iron, hinders the precipitation of graphite, increases the supercooling degree of molten iron, promotes the whitening tendency, and reduces the vulcanization reaction. The content of free sulfur suppresses its negative effects, and the generated sulfur inclusions are heterogeneous crystal nuclei precipitated by graphite, which play an important role in the stability of graphite morphology.